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1. Digestive System

2. All living organisms require food for energy and
organic matters for growth. Food is organic
compound and usually of plant or animal origin.
The major components of our food are
carbohydrates, proteins and fats. It also contains
vitamins and minerals in small quantities. The
process of conversion of complex food substances
to simple absorbable forms is called digestion and
is carried out by our digestive system by
mechanical and biochemical methods. The
digestive system also absorbs water, vitamins and
minerals, and eliminates wastes from the body.

3. The human digestive system includes the
gastrointestinal tract (or alimentary canal or
digestive tract) and the associated glands.
The gastrointestinal tract is a continuous tube
that extends from the mouth to the anus. The
organs of the gastrointestinal tract include
the mouth, pharynx, esophagus, stomach,
small intestine and large intestine. The
associated glands include salivary glands,
liver, gallbladder and pancreas.

4. The digestive system performs six basic
digestive processes:
Ingestion -Taking food into the mouth.
Secretion - Release of acid, enzymes and bile
into the lumen of the gastrointestinal tract,
Motility-Refers to propulsive and mixing
movements which cause propulsion and mixing
of food through the gastrointestinal tract.
Digestion - Mechanical and chemical
breakdown of food.
Absorption- Assimilation of digested products
from the gastrointestinal tract into the blood
and lymph.
Defecation- The elimination of feces from the
gastrointestinal tract.

5. Gastrointestinal tract
Organs of the gastrointestinal tract include
the mouth, pharynx, esophagus, stomach,
small intestine and large intestine. The
length of the alimentary canal is about 5-7
meters. In the gastrointestinal tract, several
muscle rings known as sphincters present,
which restrict the flow of contents to
optimize digestion and absorption.

6. Muscularis
mucosae
Lamina propria
Mucous epithelium
Lymphatic
nodule
Sub
mucosa
Gland
Plexus
Nerve
vessels
Connective
tissue layer
Peritoneum
Circular
muscle layer
Longitudinal
muscle layer
Duct from
gland
Gland Mesentery
Mucosa
Plexus
Muscularis
Mucosa:The mucosa is the inner lining
of the lumen of the gastrointestinal tract.
It is the absorptive and major secretory
layer. It is a mucous membrane
composed of connective tissue and
smooth muscles

7. Submucosa:A layer of dense
irregular connective tissue that
surrounds the mucosa. It has large
blood vessels and lymphatic
vessels.A nerve network known
as the submucosal plexus
(Meissner's plexus) lies within the
submucosa (plexus means
'network').
Muscularis
mucosae
Lamina propria
Mucous epithelium
Lymphatic
nodule
Sub
mucosa
Gland
Plexus
Nerve
vessels
Connective
tissue layer
Peritoneum
Circular
muscle layer
Longitudinal
muscle layer
Duct from
gland
Gland Mesentery
Mucosa Plexus
Muscularis

8. Muscularis propria (or muscularis externa): It is
dominated by smooth muscle cells in two layers
an inner circular layer and outer longitudinal
layer - that play an essential role in mechanical
processing and in the movement of materials
along the gastrointestinal tract. Between
inner circular layer and an outer
longitudinal layer another nerve
network known as the
myenteric plexus
(Auerbach's plexus),
lies.
Muscularis
mucosae
Lamina propria
Mucous epithelium
Lymphatic
nodule
Sub
mucosa
Gland
Plexus
Nerve
vessels
Connective
tissue layer
Peritoneum
Circular
muscle layer
Longitudinal
muscle layer
Duct from
gland
Gland Mesentery
Mucosa
Plexus
Muscularis

9. Serosa (serous membranes):The outer
connective tissue covering of the digestive tract
is the serosa, which secretes a watery, slippery
fluid that lubricates and prevents friction
between the digestive organs and the
surrounding viscera.
Muscularis
mucosae
Lamina propria
Mucous epithelium
Lymphatic
nodule
Sub
mucosa
Gland
Plexus
Nerve
vessels
Connective
tissue layer
Peritoneum
Circular
muscle layer
Longitudinal
muscle layer
Duct from
gland
Gland Mesentery
Mucosa
Plexus
Muscularis

11. The small intestine has three distinct regions
– the duodenum, jejunum, and ileum. The
duodenum, the shortest, is where preparation
for absorption through small finger-like
protrusions called villi begins.
The jejunum is specialized for the absorption
through its lining by enterocytes: small
nutrient particles which have been previously
digested by enzymes in the duodenum. The
main function of the ileum is to absorb vitamin
B12, bile salts, and whatever products of
digestion were not absorbed by the jejunum.
Duodenum
Jejunum
Ileum

12. Many of the Digestive enzyme that
act in the small intestine are
secreted by
the pancreas and liver and enter the
small intestine via the pancreatic
duct. Pancreatic enzymes
and bile from the gallbladder enter
the small intestine in response to
the Hormone cholecystokinin,
Secretin promotes the release
of bicarbonate
into the duodenum in order to
neutralize the potentially harmful
acid coming from the stomach.

13. • Digestion in the Mouth
• Food is broken down mechanically by teeth
and chemically by certain enzymes.
• Mastication of food with saliva(water, mucous,
ptyalin, maltase) acts on starch. It splits starch
and other complex carbohydrates into maltose
which is broken down by maltase into glucose.
• Salivary a-amylase occurs as a mixture of
closely related isoenzymes. Each is a single
chain polypeptide to which is attached an
oligosaccharide.
• In humans, saliva also contains an enzyme,
maltase in traces. It splits starch and other
complex carbohydrates into maltose which is
broken down by maltase into glucose.

14. • The gastric glands in the wall of the stomach secrete
gastric juice
• The gastric glands consist of 3 kinds of cells: chief cells
filled with secretory, granules containing pepsinogen,
Oxyntic (parietal) cells which secrete hydrochloric acid,
and mucous cells which secret mucous.
• The fasting stomach of humans secretes about 8 to 15 ml
of gastric juice in 24 hr. gastric juice is composed of water,
mucin, salts, hydrochloric acid and three enzymes, pepsin,
rennin and fat splitting gastric lipase.
• The pepsin is secreted in the form of its zymogen
precursor pepsinogen contact with food. and rennin in the
form of prorennin.
• The hydrochloric acid performs several functions in the
stomach. First, it kills several bacteria. It converts
pepsinogen into active pepsin and provides an acidic
medium for the action of pepsin.

15. Pepsin : It is a protein-splitting or proteolytic
enzyme.The precursor pepsinogen is
converted into pepsin both by the acidity of
the gastric juice and by pepsin itself. Pepsin
digests proteins up to the stage of peptone
Rennin It acts by splitting the milk protein
(caseinogen) into two smaller fragments called
casein and whey (the watery part of milk).The
casein combines with the calcium usually found
in milk to form an insoluble calcium protein or
curd.
Lipase :The lipase of the gastric juice of some
animals is a fat-splitting enzyme, and acts on
the fats which are in very fine emulsion
Digestion in the Small Intestine From the
stomach, the semi digested or semi-liquid food
passes into the small intestine

16. • Pancreatic juice
It contains sodium carbonate and the following
enzymes
• (a)Trypsin :This is secreted in an inactive form
called trypsinogen and is activated by another
enzyme enteropeptidase (previously called
enterokinase) from the intestinal wall.Trypsinogen
consists of a single peptide chain and its conversion
into trypsinTrypsin acts upon natural proteins and
splits them to proteases and peptones.
• (b) Chymotrypsin :This is also secreted in an
inactive form known as chymotrypsinogen, and is
activated into chymotrypsin by trypsin.
Chymotrypsin also acts upon proteins, converting
them into peptides, and clots milk.

17. • Pancreatic lipase or steapsin : It
is a lipolytic enzyme and converts
fats into fatty acids and glycerol.The
mechanism of pancreatic lipase is
unique.The main products of its
action on triacylglycerol's are the
fatty acids and the 2-monoacyl
glycerol and each enzyme molecule
can split 4 * 10 ester bonds per
minute.Thus, the substrate for its
action is present in lipid phase
• These are bile salts and a colipase
secreted into the pancreatic juice.
Bile salts help to emulsify fat into
particles with an increased total
surface area available for lipase

18. (8) Other pancreatic enzymes : In addition to the
above enzymes, pancreatic juice also contains maltase
and ribonuclease The former splits maltose into
glucose, whereas the latter is responsible for the
hydrolysis of RNA to smaller nucleotides. Pancreatic
juice also contains esterases that can hydrolyze short-
chain fatty acid esters, eg. tributyrin.These esterases
have an absolute requirement for bile salts.There is
also a cholesterol esterase, which hydrolyzes
cholesterol esters to fatty acids and cholesterol. Pro
Phospholipase A2 is produced in the pancreas and is
activated by trypsin produce phospholipase A2, which
is active only in the presence of Ca2+ and bile salts.
The enzyme liberates a fatty acid from the 2 position
of phosphatidylcholine to produce lysophosphatidyl
choline.The latter aids in emulsification of the dietary
lipid.

19. • (ii) Intestinal juice In addition to the
enterokinase mentioned above, the
succus entericus (intestinal
secretion) contains several enzymes
which are necessary to complete the
gastric digestion. One of these,
erepsin, completes the gastric
digestion of peptones and proteoses,
and splits them into amino acids.The
other enzymes are concerned with
carbohydrate splitting. Maltase
converts maltose into glucose,
Sucrase splits sucrose to glucose and
fructose, and lactase breaks down
lactose (milk sugar) into glucose and
galactose.The acid contents from
the stomach also cause the intestinal
mucosa to release the hormone,

20. • secretin, which carried in the blood to the
pancreas, stimulates the flow of pancreatic juice.
• The intestinal juice also contains some peptidases,
(aminopeptidases, dipeptidases) which complete
the digestion of various peptides to amino acids.
• In addition, a lipase as well as a number of
enzymes for the hydrolysis of nucleic acid
(nucleases, nucleotidase, nucleosidase) also occur
in the intestinal juice.
• The intestinal juice also contains enteropeptidase
and alkaline phosphatase are actually secreted by
the intestinal cells.
• These enzymes complete the digestion of the
various nutrients at the surface or within the
cytoplasm of the intestinal epithelial cells.

21. (iii) Bile
• It is made up of water, bile salts and certain
pigments stored in the gall bladder and is
stimulated to flow down the common bile
duct when food enters the small intestine.
• A hormone, cholecystokinin, regulating the
flow of bile because it stimulates
contraction of the gall bladder.
• Bile salts emulsify fat into small particles so
that the fat-splitting enzymes may have a
chance to reduce them. By virtue of its
alkaline nature bile creates a suitable
medium for intestinal digestion.The bile salts
are formed in the liver cells in a manner
analogous to the formation of hippuric acid :

22. Water
Mucin
Inorganic ion
HCL
Pepsin
Lipase
renin
Gastrin
Secretin
Pancreozymin
Intestinal juice
Water
Enterokinase, erepsin,
maltase
Sucrase, lactase, peptidase,
nucleases
Intestine
Simple sugar
Amino acid, fatty acid
Pancreatic juice
Water
Inorganic ion
Trypsin, Chymotrypsin
Corboxypeptidase
Amylase, lipase, maltase, ribonuclease
Bile
Water, bile salts,
pigments,
cholesterol
Cholecystokinin
Food
Carbohydrate
Fat
protein
Saliva
Water, mucin,
amylase
Enterocrinine
duocrinin
Liver
Physiology of
mammalian
digestion

23. Hydrophobic
Hydrophilic
Bile Salt
Emulsified
Fat Droplets
Fat Globules

24. Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by
the pancreas and cleave proteins into smaller peptides. Carboxypeptidase, which is a
pancreatic brush border enzyme, splits one amino acid at
time. Aminopeptidase and dipeptidase free the end amino acid products.
Lipids (fats) are degraded into fatty acids and glycerol. Pancreatic lipase breaks
down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with
the help of the salts from the bile secreted by the liver and stored in the gall bladder. Bile
salts attach to triglycerides to help emulsify them, which aids access by pancreatic
lipase. This occurs because the lipase is water-soluble, but the fatty triglycerides are
hydrophobic and tend to orient towards each other and away from the watery intestinal
surroundings. The bile salts emulsify the triglycerides in the watery surroundings until the
lipase can break them into the smaller components that are able to enter the villi for
absorption.

25. Some carbohydrates are degraded into simple sugars, or monosaccharides (e.g., glucose). Pancreatic
amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Other carbohydrates
pass undigested into the large intestine and further handling by intestinal bacteria. Brush border enzymes
take over from there. The most important brush border enzymes are dextrinase and glucoamylase, which
further break down oligosaccharides. Other brush border enzymes are maltase, sucrase and lactase.
Lactase is absent in some adult humans and, for them, lactose (a disaccharide), as well as most
polysaccharides, is not digested in the small intestine. Some carbohydrates, such as cellulose, are not
digested at all, despite being made of multiple glucose units. This is because the cellulose is made out of
beta-glucose, making the inter-monosaccharidal bindings different from the ones present in starch, which
consists of alpha-glucose. Humans lack the enzyme for splitting the beta-glucose-bonds, something
reserved for herbivores and bacteria from the large intestine.

26. Intestinal
Villi
Small
Intestine
Lacteal
Blood capillary
network
Enteroendocrine cell
Digested food is now able to pass into the blood vessels
in the wall of the intestine through either diffusion or active
transport. The small intestine is the site where most of the
nutrients from ingested food are absorbed. The inner wall,
or mucosa, of the small intestine, is lined with simple
columnar epithelial tissue.
Absorption of the majority of nutrients takes place
in the jejunum, with the following notable
exceptions:
•Iron is absorbed in the duodenum.
•Folate (Vitamin B9) is absorbed in the duodenum
and jejunum.
•Vitamin B12 and bile salts are absorbed in
the terminal ileum.
•Water is absorbed by osmosis and lipids by
passive diffusion throughout the small intestine.
•Sodium bicarbonate is absorbed by active
transport and glucose and amino acid co-transport
•Fructose is absorbed by facilitated diffusion.